1. Field of the Invention
The present invention relates to the sector of continuously variable hydrostatic transmissions with torque division. It relates to a transmission of this kind in accordance with the preamble of claim 1.
2. Discussion of Related Art
Torque-split transmissions, especially for use on agricultural or construction vehicles such as tractors, have long been known. In such torque-split transmissions, the power present at an input shaft or drive-input shaft, which is generally output by an internal combustion engine, is divided between a first, mechanical power branch with a fixed transmission ratio and a second power branch with a continuously variable transmission ratio and is then combined again so as to be available at an output shaft or drive-output shaft. The second power branch is generally designed as a hydrostatic branch, in which two hydrostatic axial-piston machines (hydrostatic units) of the oblique-axis or swashplate type, which are hydraulically interconnected, operate optionally as a pump or a motor. Here, the transmission ratio can be varied by changing the pivoting angle of the cylinder block or of the swashplate. Dividing the power between the two power branches and combining the divided powers is generally performed by means of a planetary transmission. Various embodiments of torque-split transmissions of the type described are disclosed in DE-A1-27 58 300, DE-C2-29 04 572, DE-A1-29 50 619, DE-A1-37 07 382, DE-A1-37 26 080, DE-A1-39 12 369, DE-A1-39 12 386, DE-A1-43 43 401, DE-A1-43 43 402, EP-B1-0 249 001 and EP-A2-1 273 828.
In order to be able to use a torque-split transmission successfully in practice, it should in general terms be distinguished by the following characteristics:                The transmission should have a high efficiency over the entire speed range. This should be the case especially at the high speeds of travel that occur over long periods in road traffic.        The transmission should be of compact construction to enable it to be installed in a very wide variety of vehicles, if possible without any design limitations.        The transmission should allow full electronic control in conjunction with the engine management system and should provide emergency running programs that are adequate even if certain control elements fail.        The transmission should allow high powers to be transmitted.        The construction of the transmission should be as simple as possible in order to limit power losses and increase reliability of operation.        The transmission should allow full electronic control in conjunction with the engine management system and should provide emergency running programs that are adequate even if certain control elements fail.        
DE-A1-43 43 402, which was mentioned at the outset, has already described a continuously variable hydrostatic torque-split transmission (referred to in German as an SHL-Getriebe (Stufenloses Hydrostatisches Leistungsverzweigungsgetriebe)) which is distinguished by two hydraulically coupled oblique-axis hydrostatic units of the same type which can be coupled in different ways to a planetary differential transmission by way of clutch pairs or selector elements K1/K2 and K3/K4 respectively. The known continuously variable hydrostatic torque-split transmission has been used and tested in city buses under the type designation SHL-Z. The two hydrostatic units employed have a pivoting range of just 0-25°. For forward travel, this gives 3 drive positions or drive ranges: in the first drive range, the hydrostatic component of the power transmitted is 100% at the starting point and then decreases linearly with speed toward zero. In the second drive range, it goes from zero to a maximum of about 27% and then falls back to zero again. In the third drive range, it goes from zero to a maximum value of 13% at the maximum forward speed.
The hydrostatic power transmission branch in such a transmission generally comprises two hydrostatic axial-piston machines, which are connected to one another hydraulically and of which one operates as a pump and the other as a motor. The two machines can swap roles depending on the drive position.
The hydrostatic axial-piston machines form a significant part of the hydrostatic torque-split transmission and have a decisive effect on the characteristics of the transmission, such as efficiency, overall size, complexity, the speed range covered, the type and number of drive positions and the like. Examples of hydrostatic axial-piston machines of this kind are disclosed in DE-A1-198 33 711, DE-A1-100 44 784 or US-A1-2004/0173089. The mode of operation and theory of hydrostatic axial-piston machines and of a torque-split tractor transmission fitted therewith are described in a publication by the Technical University of Munich from the year 2000 by H. Bork et al., entitled “Modellbildung, Simulation and Analyse eines stufenlosen leistungsverzweigten Traktorgetriebes” [Modeling, Simulation and Analysis of a Continuously Variable Torque-split Tractor Transmission].
Finally, WO-A1-2006/042434 has disclosed a continuously variable hydrostatic torque-split transmission which operates with two large-angle hydrostatic units with a pivoting range of up to 50°. The overall speed range of the transmission is divided into two sections, in which the two hydrostatic units swap roles as pump and motor. For the switchover between the two sections, two dual clutches are provided, by means of which the hydrostatic units are connected in different ways to the torque-splitting planetary drive. With this transmission, it is possible to cover a large speed range in a continuously variable manner while achieving high efficiency at the same time. However, it is of relatively large size because space is required for the switchable connections involving the dual clutches.